Meibomian glands are a type of sebaceous gland located in the tarsal plate of the upper and lower eyelids. These glands are responsible for the supply of meibum, an oily substance that prevents evaporation of the eye's tear film, prevents tear evaporation, and makes the closed lids airtight. There are approximately 50 glands on the upper eyelids and 25 glands on the lower eyelids. Meibomian glands are distinguished by grape-like clusters of acini on the mucocutaneous lid junction and empty their lipid content (meibum) at this junction in order to coat the ocular surface via holocrine secretion. The glands are anchored by cells that produce both polar and nonpolar lipids, which are then stored in lysosomes that merge into larger storage granules. As these cells continue to enlarge with lipid, they eventually commit apoptosis and rupture to release the meibum into the meibomian gland opening and spill the meibum over the ocular surface. Meibum is fluid at the temperature of ocular surface, and distributed over the ocular surface in a thin, smooth film on top of the aqueous layer. This lipid layer prevents evaporation of the aqueous layer. Alterations in the meibum composition, property and level can have a profound effect on the health of the eyelid margins and ocular surface. Meibomian gland dysfunction in the general population is pretty high with some estimates as high as 39% with an increase in incidence in contact lens wearers.
Meibomian gland secretions form the lipid layer of tears and consist of polar and nonpolar lipids. The lipid composition of the meibum can affect tear parameters like initial formation of a composite monolayer with polar and nonpolar phases, adequate fluidity near body temperature, and the ability to undergo compression and expansion during blinking. These properties are very important for effective polar lipid structuring and fluidity (melted physical state) at normal body temperature. Any alterations in the saturation of the fatty acids can lead to instability of tears.
MGD patients usually have normal production of aqueous tears by their lacrimal glands, their meibomian glands can atrophy and this is frequently accompanied by metaplasia of the ductal epithelium of these glands. Anterior erosion of the mucocutaneous junction of the eyelid is often noted, as well as eyelid and conjunctival infection, eyelid margin irregularity, corneal epithelial changes, and corneal vascularization. In some cases, abnormal overproduction of meibum can also cause the same problems.
As meibomian glands lining the eyelids produce lipids that promote the stability of the tears and reduce evaporation of the tear film, dysfunction of the meibomian glands can lead to lipid insufficiency that destabilizes the tear film and causes decreases in tear film break-up time and evaporative dry eye.
MGD may also be characterized by increased melting point of the lipids, causing solidification of the lipids and obstruction of the meibomian gland secretion. This can result in cysts, infections and decreased lipid content in the tears. MGD is also characterized by excess, abnormally turbid secretion that gets inspissated and plugs the meibomian orifices. This is followed by metaplasia of the meibomian ducts (abnormal hyper-keratinization). Blockage and resistance to flow results in inflammation and vascularization (redness) of tissues around the orifice. Inflammatory mediators accumulate in the tear film leading to damage of ocular surface. Sequalae of all these events is inflammatory scarring of the duct leading to stenosis. Initially glands swell and eventually atrophy.
Common complaints of MGD patients include blurred or filmy vision, light sensitivity or photophobia, burning or foreign body sensations in the eye, excessive tearing, intolerance to contact lens and pain.
Light sensitivity is an intolerance of sunlight, fluorescent light or incandescent light. It is one of the symptoms of MGD and is associated with other conditions such as corneal abrasion, uveitis, meningitis, a detached retina, contact lens irritations, sunburn or refractive surgery.
Blurred vision is a lack of sharpness of vision, and may result from abnormalities such as nearsightedness, farsightedness, presbyopia, and astigmatism. It is also associated with MGD and other ocular surface conditions.
Currently, various lipid based artificial tears or lubricants are used to alleviate the symptoms of MGD. Physical treatments, such as maintaining good hygiene, heating, and massage, are often used as well. However, these treatments do not cure the disease. There is a need for an effective and safe treatment for MGD.